In the float process for forming flat glass, a stream of molten glass is delivered onto a pool of molten metal (usually tin) within a float forming chamber where the molten glass is formed into a ribbon of desired thickness. In controlling the float forming process, it is desirable to monitor precisely the location of the edges of the glass ribbon floating on the molten metal. Because of the high temperatures involved in the float forming process (typically about 600.degree. C. to 1100.degree. C.) and the presence of heating means within the forming chamber, the glass and the tin and interior elements of the forming chamber emit a considerable amount of visible radiation, thereby rendering optical detection of the glass ribbon edge difficult. In other words, the brightness of the float chamber interior often obscures the glass/tin interface. Light filtering and polarizing devices have been employed to render the glass/tin interface observable, but contrast between the glass and the tin surfaces is not as great as would be desired.